The present invention relates to manufacturing brake disks out of composite material, and in particular to preparing fiber preforms for such manufacture. The field of application of the invention is more particularly that of brake disks made of thermostructural composite material. Thermostructural composite materials for brake disks are typically carbon-carbon or xe2x80x9cCxe2x80x94Cxe2x80x9d composites constituted by a reinforcing preform of carbon fibers densified by a carbon matrix and optionally subjected to final siliciding treatment. Other suitable composite materials are ceramic matrix composites or xe2x80x9cCMCsxe2x80x9d constituted by a reinforcing preform of refractory fibers (carbon or ceramic) densified by a ceramic matrix, e.g. carbon-silicon carbide or xe2x80x9cCxe2x80x94SiCxe2x80x9d composites.
The use of thermostructural composite materials, in particular Cxe2x80x94C composites, for making brake disks is well known, in particular for the multi-disk brakes of airplanes, and also for land vehicles, e.g. F1 racing cars.
The usual technique for manufacturing such disks consists in making annular fiber preforms and in densifying them with a matrix of carbon that fills the pores of the preforms.
The preforms are usually made by superposing layers of fiber fabric which are bonded together, in particular by needling, so as to give the preform the cohesion it requires to avoid any risk of a disk delaminating while it is in use. The fiber fabric layers are typically multidirectional two-dimensional layers formed at least in part out of continuous filaments, e.g. layers made by weaving or braiding or knitting threads made up of continuous or discontinuous fibers, or layers made up of a plurality of sheets of unidirectional cables disposed in different directions and bonded together by light needling. Fiber webs or layers of felt can be added to provide discontinuous fibers that are easily taken by the needles during needling to provide Z-direction bonding between layers (i.e. transversely relative to the faces of the layers). These fiber webs or felt layers also serve to recycle the fiber scrap that is produced when cutting the fiber fabric layers as is necessary to obtain annular preforms.
The use of fiber webs or felts made from such scrap material is described in particular in documents FR-A-2 626 294 and EP-A-0 530 741. According to the latter document, the layers of felt can be interleaved between the layers of fiber fabric in the core of a preform, or they can be added to the faces of the core in order to constitute surface layers of a preform that are designed to be eliminated in machining operations that take place during and/or after densification.
Preforms can be densified by chemical vapor infiltration or by using a liquid, both of which techniques are well known. Chemical vapor infiltration consists in placing the preforms that are to be densified in an enclosure into which a matrix-precursor gas is admitted that, under controlled conditions of temperature and pressure, diffuses within the preform and forms a deposit of matrix material on the fibers by reaction between its own components or by decomposition. When deposition takes place preferentially in the surface pores of the preform, tending to close them prematurely, it can be necessary to proceed with one or more intermediate surface-machining or xe2x80x9cdescalingxe2x80x9d operations in order to recover surface pores and allow densification to proceed to the core of each preform.
Densification by means of a liquid consists in impregnating a preform with a matrix precursor in the liquid state, e.g. a resin, and then transforming the precursor, generally by heat treatment. Several consecutive impregnation cycles can be necessary in order to achieve the desired degree of densification. It is also possible to combine the techniques of chemical vapor infiltration and of liquid impregnation.
Compared with metal disks, brake disks made of thermostructural composite material, and in particular of Cxe2x80x94C composite material, provide a considerable saving in mass while providing excellent tribological properties and low wear. They are also well adapted to the severe conditions of use encountered in airplanes and in F1 racing cars.
Extending the use of thermostructural composite brake disks to other types of vehicle, such as trains, heavy trucks, coaches, utility vehicles, or private cars is being slowed down specifically because of particular problems encountered in such use.
Thus, tests performed by the Applicant on a top-of-range private car with Cxe2x80x94C composite brake disks made using a method analogous to that used for manufacturing airplane brake disks have demonstrated that they can sometimes give rise to undesirable vibration, and to braking torque that can be irregular. In those brake disks, the preforms were made by needling together layers of base texture, which texture was made up of a plurality of unidirectional sheets of cables disposed at different angles (e.g. three sheets at 0xc2x0, +60xc2x0, and xe2x88x9260xc2x0 C.) that were themselves preneedled together. It is likely that the use of that base fabric gives rise to irregular wear of the friction faces of the disks that come into contact with the brake pads, which phenomenon gets worse over the lifetime of the brake disks and generates vibration.
An object of the present invention is to provide a method of preparing fiber preforms that enables brake disks to be made out of composite material that do not present those drawbacks.
A particular object of the present invention is to provide such a method enabling brake disks to be made of composite material that are suitable for use on industrial or private motor vehicles without generating undesirable vibration and regardless of braking conditions, while also delivering braking torque that is regular and without any wear that is abnormally fast.
Another object of the present invention is to obtain such performance at a cost price that is compatible with the brake disks being used in mass-produced industrial or private motor vehicles.
Such objects can be achieved by a method of the type comprising superposing and bonding together fiber layers comprising structural layers formed at least in part out of continuous filaments and out of at least one felt layer, in which method structural fiber layers are used to form at least a first preform portion that is to constitute the fiber reinforcement of the core of the brake disk, while the or each preform portion that is to constitute the fiber reinforcement of a friction portion of the brake disk is constituted by a felt, at least in its portion adjacent to the friction face.
The term xe2x80x9cstructural fiber layer formed at least in part out of continuous filamentsxe2x80x9d is used herein to mean a layer that is woven, braided, or knitted out of continuous threads, themselves made of continuous or discontinuous fibers, or a layer constituted by a sheet of unidirectional, continuous cables, twisted strands, or threads, the cables, strands, or threads, themselves being constituted by continuous or discontinuous fibers, or else a layer constituted by a plurality of such sheets superposed in different directions and bonded together, e.g. by preneedling, or indeed such a fiber layer associated with a thin web of fibers to which it is bonded, e.g. by light needling. Such structural fiber layers are used to constitute a preform portion that is suitable for conferring to the core of the brake disks the mechanical properties which are required for transmitting braking forces without rupturing or damaging the disks, in particular where the core is mechanically linked to the member with which the disk is bound in rotation. The structural fiber layers can be placed flat, parallel to the faces of the disks, or they can be wound around the axis of the disks. If they are wound, the portion of the preform corresponding to the core of the disk can be obtained by cutting slices from a sleeve obtained by rolling up a strip of structural fiber fabric on a mandrel to form layers that are superposed on one another.
The felt forming, at least a part of the or each portion of the preform that is to constitute the fiber reinforcement of a friction portion of the brake disk is in the form of at least one relatively thick layer having fibers at a low volume density, preferably less than 20%, where fiber volume density is the fraction of the apparent volume of the felt that is actually occupied by the fibers. The term xe2x80x9crelatively thick felt layerxe2x80x9d is used herein to mean a felt which, in the prepared preform, is of a thickness that is not less than about 1 mm. After densification, the major portion of the friction lining in the vicinity of the friction face is constituted by the composite material matrix. Typically, the friction lining in the vicinity of the friction face has 10% to 15% by volume of fibers, 65% to 75% by volume of matrix, and 15% to 20% by volume of residual open pores.
By having a preform of this structure, and in particular by having felt present in the vicinity of the friction face, no undesirable vibration appears during braking, contrary to that which has been observed with brake disks in which the preform is constituted by needled structural fiber layers, even in the friction portions. Such vibration can be due to wear of the friction face that becomes irregular in the long term. The presence in the friction portion of felt, i.e. of non-oriented short fibers, instead of structural fiber layers, and also the occupation of the majority fraction by the matrix, give rise to smaller anisotropy and less rigidity, thus avoiding the appearance of wear irregularities, or promoting attenuation thereof.
In addition, it has been observed that the braking torque is remarkably regular. Furthermore, the performance obtained is just as good in a wet environment as it is in a dry environment.
It can be envisaged to use felt not only for the or each portion of the preform that corresponds to a friction portion of a disk, but also to form preferably thin layers that are interleaved between structural fiber layers in the first portion of the preform that corresponds to the core of the disk. When structural fiber layers are disposed parallel to the faces of the disk, this contributes to imparting a certain amount of flexibility to the disk in the axial direction and increases its capacity to absorb vibration.
The layers constituting the first portion of the preform corresponding to the core of the disk are preferably bonded together by needling. The felt constituting at least in part the or each portion of the preform corresponding to a friction portion of the disk can be formed as a single layer or as a plurality of superposed layers that are likewise advantageously bonded together by needling. Bonding between the felt and the first portion of the preform can also be performed by needling. It should be observed that under such circumstances the felt must be needled without being compressed in such a way as to increase the fiber volume density above the desired maximum.
An annular brake disk preform can be made from plane fiber layers either by superposing and bonding together fiber layers that are precut to an annular shape, or else by superposing and bonding together optionally circular fiber layers without any center holes, and subsequently cutting the preform through all of the superposed and bonded together fiber layers. It is also possible to make the portion of the preform that corresponds to the core of the disk by winding a fiber fabric as superposed layers which are bonded together, while the or each portion of the preform corresponding to a friction portion of the disk is made by superposing and bonding together fiber layers that are plane.
According to another aspect, the invention also provides a method of manufacturing brake disks out of composite material by densifying preforms prepared in the manner given above.
Advantageously, to manufacture an assembly comprising both a central rotor brake disk having two opposite friction faces and also two end stator brake disks having one friction face each, e.g. for an industrial vehicle disk brake (heavy truck or coach), four substantially identical component preforms are made, each having a first portion corresponding to a core portion and a second portion corresponding to a friction portion, the preforms are densified, and the rotor disk is obtained by putting two densified preforms together via their faces opposite from their friction faces. This means that rotor disks and stator disks become different only after they have been densified. It is also possible to envisage assembling two component preforms prior to densification in order to obtain a rotor disk preform, in which case rotor disks and stator disks become different after the preforms have been prepared, but before densification.
According to yet another aspect, the invention provides disk brakes manufactured from preforms prepared in the manner given above.